1. Field of the Invention
This invention relates to doping of semiconductor material and more particularly to a method and apparatus for controlling the doping of a body of single-crystal semiconductor material to a desired heavy concentration of a highly volatile dopant such as, for example, indium.
2. Description of the Prior Art
High-purity single-crystal silicon heavily doped with indium is useful as detector material. Such material has been made satisfactorily in float-zone crystal growers. In the prior-art process, a rod of purified polycrystalline silicon about thirty to forty centimeters long is prepared for doping by removing a core about ten centimeters long from one end of the rod. After cleaning the resulting hole with an appropriate acid to remove contaminants, a charge of indium is placed therein. The amount of indium used in the charge is empirically determined to produce a heavy dopant concentration between desired limits in at least a portion of the subsequently processed rod. After the dopant is in place, the hole is closed with a plug of purified silicon. The rod of silicon is then mounted in a float-zone crystal grower and passed through the crystal grower's heating coil, dopant-charged end first, to grow a single crystal of indium-doped silicon.
A substantial amount of high-quality indium-doped silicon has been made by the prior-art procedure described above. However, the procedure has several disadvantages which tend to make it relatively costly and time consuming.
One disadvantage is that not all of a rod of silicon of reasonably practical length can be doped to the required heavy concentration of indium within a preselected specified narrow range of limits by this procedure. For example, when a thirty-centimeter rod of silicon is so processed, only about fifteen centimeters of the rod will be found to have dopant concentration within the specified range of limits. The dopant concentration in the remainder of the silicon will be too low. Thus, about half of the silicon in the rod must be reprocessed to make it suitable for use as indium-doped detector material. The concentration of indium in the silicon rod falls below the lower specified limit because indium is so highly volatile. A significant portion of the original charge of indium simply vaporizes out of the melt zone and is thereby lost to the process.
Another disadvantage of the prior-art procedure arises from the extensive handling and tooling required to prepare the polycrystalline rod for doping. Extreme care is required to prevent contamination of the silicon with undesired impurities during the steps of coring, charging with dopant and plugging. Although cleaning of the tooled silicon with acid is usually successful, there remains a risk that the resulting doped silicon will not meet specifications because of contamination due to the tooling and handling.
A third disadvantage of the prior-art procedure described above arises from the fact that, on occasion, a rod of polycrystalline silicon being doped and converted to single-crystal structure will lose that structure at some point during processing. When this occurs, the procedure cannot be restarted since the amount of indium remaining in the silicon is insufficient to produce the desired heavy dopant concentration. The rod must be removed from the crystal grower, recored, recleaned, recharged with dopant and then replugged before another attempt can be made to convert it to doped, single-crystal material. This additional handling requires a considerable amount of extra time and expense. The additional handling also increases the risk that the rod will be contaminated as discussed above.
Thus, there is a need for an improved approach to producing single-crystal rods of silicon doped with indium to heavy concentrations of dopant within specified preselected limits. The improved approach will preferably be capable of producing, as desired, either a uniform dopant concentration along the length of a rod or a desired variation of dopant concentration. In order to accomplish this, it is important to be able both to vary and to control the amount of dopant which is available for doping a given portion of the semiconductor rod. The improved approach will preferably require no tooling of the silicon rod and only a minimum of handling. And, on those occasions when the single-crystal structure of the silicon rod breaks down during processing, the improved procedure will preferably be capable of being restarted for the same pump-down of the crystal grower without demounting the silicon rod from the crystal grower.
Procedures and apparatus for doping silicon which use a dopant-containing gas continuously fed through tubing to the vicinity of the melt zone in a float-zone crystal grower would appear to have some of the desired advantages mentioned above. Examples of this approach are given in the following references: E. Enk et al, U.S. Pat. No. 3,141,848, "Process for the Doping of Silicon", July 21, 1964; Katz et al, "Gas Doping of Float Zone Silicon Crystals in Vacuum From a Solid Source Using Pressure Differential to Transport Dopant", Journal of Crystal Growth 19 (1973) 113-116; Keller, U.S. Pat. No. 3,804,682, "Method for Controlled Doping of Semiconductor Crystals", Apr. 16, 1974; and Keller et al, U.S. Pat. No. 4,126,509, "Process for Producing Phosphorus-Doped Silicon Monocrystals Having a Select Peripheral Dopant Concentration Along a Radial Cross-Section of Such Monocrystal", Nov. 21, 1978. However, it is impractical to attempt to convey gaseous elemental indium from an external source to the vicinity of the melt zone. A carrier gas could be used to transport indium, but the carrier gases known to be capable of doing this would, upon coming in contact with the molten silicon, tend to contaminate it with more carbon than is acceptable in detector grade semiconductor material.
Procedures and apparatus for doping silicon which continuously advance a small rod of heavily-doped silicon toward the melt zone of a larger undoped rod of silicon would also appear to have the capability of providing some control of the rate of doping of the larger rod of silicon along its entire length. This approach is illustrated in the following references: E. Enk et al, cited above, and Keller, U.S. Pat. No. 3,954,416, "Apparatus for Positively Doping Semiconductor Crystals During Zone Melting", May 4, 1976. A related approach is illustrated in Kohl et al, U.S. Pat. No. 4,110,586, "Manufacture of Doped Semiconductor Rods", Aug. 29, 1978. In the process of the latter reference, a small rod of heavily doped silicon is positioned with its axis substantially parallel to the axis of a large rod of undoped silicon. The two rods are fused together and jointly subjected to the zone melting process. However, these approaches appear to be incapable of being restarted during the same pump-down of the crystal grower if single-crystal structure is lost. Furthermore, in these approaches it is very difficult to control the amount of indium actually introduced into the silicon with sufficient precision. This is so for two reasons. First because the indium is initially combined with a certain amount of silicon in the small doping rod, the total amount of indium available can only be indirectly determined by measuring the resistivity of the small rod. This indirect measurement is subject to significant errors. Second, because of the high volatility of the indium, most of it will be evaporated from the small doping rod before it comes in contact with the melt zone of the larger undoped rod of silicon. Thus, a significantly larger amount of indium must be present in the small doping rod than is expected to find its way into the finished rod. The need for making available an excess of indium also reduces the precision with which the amount of indium entering the larger undoped rod can be controlled. In some cases, the amount of indium required to be made available is as much as ten times the amount of indium which eventually is fixed in the silicon.